1. Field of the Invention
This invention relates to a mobile communication system which includes a radio base station connected to an exchange to establish a communication interface with a mobile terminal such as a car telephone set.
2. Description of the Related Art
In recent years, as the employment of digital processing in car telephone systems proceeds, amplification apparatus of the type wherein a plurality of RF (radio frequency) carriers are commonly amplified are progressively employed as amplification apparatus installed in a radio base station. Where an amplification apparatus of the type mentioned is employed, a saturation output which satisfies intermodulation distortion and spurious standards is required, and the power consumption of the amplification apparatus is very high. Therefore, power consumption must necessarily be reduced.
FIG. 7 shows a mobile communication system, and FIG. 9 illustrates communications in the mobile communication system shown in FIG. 7. Referring to FIGS. 7 and 9, an exchange 21 accommodates a plurality of radio base stations 23.
A mobile terminal 22 as represented by a car telephone set is accommodated in one of the radio base stations 23 by way of which optimum communications are available with the mobile terminal 22.
The radio base station 23 establishes a communication interface between the exchange 21 and the mobile terminal 22. In particular, the radio base station 23 has three pairs of transmission antennae 37-1, 37-2 and 37-3 and reception antennae 37-4, 37-5 and 37-6 and performs communications with the mobile terminal 22 in accordance with the space diversity system. To this end, the radio base station 23 includes a modulation and demodulation apparatus (modem) 24, an amplification apparatus 25, and a transmission apparatus 39 having a multiplexing function and a demultiplexing function.
The modulation and demodulation apparatus 24 modulates and sends a transmission signal to the amplification apparatus 25 and demodulates a reception signal from the amplification apparatus 25. The modulation and demodulation apparatus 24 includes three modulation and demodulation apparatus 24a-1, 24a-2 and 24a-3 and a base band interface section 32. The modulation and demodulation apparatus 24a-1, 24a-2 and 24a-3 are individually provided for three units called sectors into which the area of directions of transmission and reception around the radio base station 23 is divided, thereby forming a three-sector configuration. Each of the modulation and demodulation apparatus 24a-1, 24a-2 and 24a-3 includes a distribution and composition section 33a, and modulation and demodulation circuits 34-1 to 34-n (n is an integral number equal to or greater than 2) provided for each one Carrier.
The distribution and composition section 33a performs composition of a transmission signal and distribution of a reception signal. Meanwhile, each of the modulation and demodulation circuits 34-i (i=1 to n) performs modulation or demodulation of a carrier frequency. Accordingly, where n modulation and demodulation circuits are involved, n carrier frequencies can be modulated or demodulated.
The amplification apparatus 25 amplifies a transmission or reception signal to a signal of a prescribed power and includes three amplification apparatus 25a-1, 25a-2 and 25a-3 individually provided for the sectors. Each of the amplification apparatus 25a-1, 25a-2 and 25a-3 includes a transmission and reception unit 38, a transmission amplification section 25b and a pair of reception amplification circuits 75a and 75b.
FIG. 8 shows details of the amplification apparatus 25a-1 as a representative of the amplification apparatus 25a-1, 25a-2 and 25a-3. Referring to FIG. 8, the transmission amplification section 25b of the amplification apparatus 25a-1 includes an input distributor 36, four amplification circuits 35-1 to 35-4 and a power composer 33.
The input distributor 36 divides a transmission signal into four divisional signals and sends the divisional signals to -the amplification circuits 35-1 to 35-4. The amplification circuits 35-1 to 35-4 are connected in parallel with each other and amplify the respective divisional transmission signals to signals of a predetermined power. The power composer 33 composes the divisional transmission signals back into a transmission signal. The transmission and reception unit 38 suppresses unnecessary frequencies of a transmission or reception signal outside a predetermined frequency band.
The amplification apparatus 25a-1 further includes a supervision control item interface circuit 76 which supervises a failure condition of the amplification circuits 35-1 to 35-4. The amplification apparatus 25a-1 further includes a reception filter 78.
Referring back to FIG. 7, the exchange 21 includes a pair of transmission apparatus 41 and 43 each having a multiplexing function and a demultiplexing function, an exchange apparatus 42, an exchange control apparatus 44 and a base station control apparatus 77.
The exchange apparatus 42 performs switching of circuits, and the exchange control apparatus 44 controls the exchange apparatus 42. The base station control apparatus 77 performs remote control of the various components in the radio base station 23.
The exchange 21 and each of the radio base stations 23 are connected to each other by way of a transmission line 45.
In the mobile communication system of the construction described above, each of the radio base stations 23 connected to the exchange 21 establishes a communication interface with a mobile terminal 22. In particular, if it is assumed that a mobile terminal 22 as represented by a car telephone set is accommodated in a radio base station 23 by way of which optimum communications are available with the mobile terminal 22 at present, then the radio base station 23 performs communications with the mobile terminal 22 in accordance with the space diversity system as it includes the three pairs Of transmission antennae 37-1 to 37-3 and reception antennae 37-4 to 37-6.
In the radio base station 23, since the modulation and demodulation apparatus 24 and the amplification apparatus 25 are provided for the individual sectors so as to have a three-sector configuration a transmission signal flows in the following manner for each sector.
In particular, a signal transferred from the exchange 21 to the radio base station 23 is sent to the modulation and demodulation apparatus 24 by way of the transmission apparatus 39. In the modulation and demodulation apparatus 24, the signal is transmitted by way of the base band interface section 32 to and divided by the modulation and demodulation circuits 34-1 to 34-n into n RF modulation waves (n multi-carriers). The carries are composed by the distribution and composition section 33a and then sent to the amplification apparatus 25. It is to be noted that the base station control apparatus 77 of the exchange 21 controls the number of carriers in response to a variation of the traffic (number of calls).
In the transmission amplification section 25b in the amplification apparatus 25a-1, the transmission signal is divided into four divisional transmission signals by the input distributor 36, and the divisional transmission signals are amplified to signals of a prescribed power by the four amplification circuits 35-1 to 35-4 and then composed by the power composer 33.
The thus composed amplified signal from the amplification apparatus 25a-1 is sent to the transmission and reception unit 38, in which unnecessary frequencies outside a predetermined frequency band are suppressed. Thereafter, the signal from the transmission and reception unit 38 is transmitted from the transmission antenna 37-1.
It is to be noted that the reason why the present system includes the parallel configuration of the amplification circuits 35-1 to 35-4 is that it employs a redundancy construction because it does not include a reserve system from the point of view of minimization and reduction of the power consumption.
Further, an RF switch is provided at an end of each of branches of the input distributor 36 and the power composer 33 so that the number of parallel connected ones of the amplification circuits 35-1 to 35-4 may be variable and a possible gain variation at the transmission amplification section 25b may not be caused upon such variation of the number.
In this manner, even if one of the amplification circuits 35-1 to 35-4, for example, the amplification circuit 35-1, fails, a variation of the output of the amplification apparatus 25a-1 is prevented. However, if the amplification circuit 35-1 fails in this manner and is disconnected from the system, then the output operation points of the other amplification circuits 35-2, 35-3 and 35-4 rise. It is to be noted that failure information of the amplification circuits 35-1 to 35-4 is collected by the supervision control item interface circuit 76.
Further, in the exchange 21, the exchange apparatus 42 performs switching of circuits under the control of the exchange control apparatus 44, and the base station control apparatus 77 performs remote control of the components in the radio base station 23.
In such a mobile communication system as described above, however, when some of a plurality of amplification circuits fail, the output operation points of the other amplification circuits rise so that the output of the apparatus may not vary. Consequently, the mobile communication system suffers from increased intermodulation distortion of the output of the apparatus. Further, in order to satisfy the intermodulation standard, an amplification circuit having a higher saturation output is required. Consequently, the mobile communication system suffers from increased power consumption by such amplification circuit.